1. Field of Invention
This invention pertains generally to the field of radio frequency metal detectors, and more particularly to the calibration of such a device.
2. Description of Prior Art
Metal detectors are used in the food processing industry, for example, to detect contaminants within a product. The unwanted material may include very small metallic particles having differing compositions. As seen in FIG. 3, the typical metal detector is housed in an enclosure 26 containing a longitudinal aperture 25 through which the product under test 23 is transported, usually by means of a conveyor belt, in the direction of arrow 24. The metal detector includes a radio frequency transducer or oscillator that radiates a magnetic field by means of some arrangement of coils that serve as a radio frequency antenna. An example of such a metal detector operating in the radio frequency range is disclosed in U.S. Pat. No. 5,994,897, entitled FREQUENCY OPTIMIZING METAL DETECTOR, issued on Nov. 30, 1999 to King.
The typical metal detector enclosure 26 includes both radiating and receiving coils formed to surround the aperture 25 through which the product travels. The oscillator coil is a continuous wire loop formed within the search head. The oscillator coil surrounds the aperture 25 and receives radio frequency excitation from an oscillator circuit. The enclosure 26 also includes an input coil connected to produce a zero input signal when no metal is present.
A disturbance in the radiated magnetic field is sensed by the input coil and processed in order to detect a metal contaminant within the product passing through the detector aperture. Modern digital signal processing techniques resolve the input signal into two signal components, one component being resistive and the other signal component being reactive. A nonzero input coil signal is due to either mechanical imbalances in the construction of the search head, inherent electrical changes in the circuitry such as frequency drift, metal being introduced into the aperture, or the effect of the product itself. The “product effect” caused by the product passing through the aperture is due primarily to electrical conduction via salt water within the product, the electrical conduction causing large magnitude resistive signals and relatively smaller reactive signals. An example of a metal detector using digital signal processing techniques is disclosed in U.S. Pat. No. 7,432,715, entitled METHOD AND APPARATUS FOR METAL DETECTION EMPLOYING DIGITAL SIGNAL PROCESSING, issued on Oct. 7, 2008 to Stamatescu.
Calibration of a metal detector is usually accomplished by the user of the detector. This process is dependent on operator skill and experience, and results in inconsistent results between different operators using the same machine. Examples of the complexity of the metal detection calibration process are disclosed in U.S. Pat. No. 6,816,794, entitled APPARATUS AND METHOD FOR DETECTING CONTAMINATION OF OBJECT BY A METAL, issued on Nov. 9, 2004 to Alvi, and in U.S. Pat. No. 7,145,328, entitled METAL DETECTOR AND ITS TEST PROCEDURE, issued on Dec. 5, 2006 to Manneschi.
FIG. 1 depicts a typical signal processing scheme used in a metal detector. The coils 1 are connected to the search head 2 that contains a radio frequency transmitter and receiver. When the coils 1 receive an electromagnetic signal the search head 2 divides the received signal into a reactive (X) component 11 and a purely resistive component 12. The signals 11 and 12 are in an analog form and so are forwarded to the analog to digital (A/D) converter 3 where the signal 11 is converted into a digital reactive component signal 13 and a digital resistive component signal 14.
Referring also to FIG. 2, the magnitude 16 of the reactive component signal 13 generated by the coils 1 when a metal contaminant is present in the item 23 as it moves through the aperture 25 is basically sinusoidal. As evidenced by the time axis 18, the elapsed time 21 (Tc) between the signal peaks 19 and 22 determines the frequency at which a particular metal detector is most sensitive to metals within the aperture 25 for a given conveyor speed. The time 21 between is dependent on the velocity of the moving item 23, the particular coil geometry and the dimensions of the aperture 25. The negative peak 22 and the positive peak 19 always occur at the same position with respect to the zero level 17 which corresponds to the geometrical center of the aperture 25. The position of the peaks 19 and 22 is dependent on a combination of coil geometry, designated by the variable Cnfg and the case dimensions Encl, which together are unique to a particular model of metal detector having an aperture 25 of a particular size. In other words, the variables Cnfg and Encl are fixed for a given metal detector. Conversely, the velocity of the conveyor transporting item 23 through the aperture 25 is virtually always variable for a given metal detector due to the specific manner in which the metal detector is operated in a given environment for a particular type of item 23.
The digitized signals 13 and 14 are sent to speed filter 30, which includes a high pass filter 4 that first receives the digitized signals 13 and 14. The frequency Fhp of the filter 4 is determined by the settings of the speed filter control 5, which calculates a control frequency Fc that is a function of the variables Cnfg and Encl as well as the conveyor speed Vc. The high pass cut off frequency Fhp is equal to 0.6*Fc. The filtered signals 27 and 28 are then processed by the signal processing algorithm 7, the resultant processed signal 29 being further filtered by a low pass filter 8. The output of the low pass filter 8 is sent to a detection algorithm 10 which ultimately determines whether or not a contaminant is present within the processed item.
The frequency FLP of the filter 8 is controlled by signal path 31 from the speed filter control 5, which causes the value of FLP to be 1.2*Fc. The appropriate value of Fc is calculated by the contaminant frequency learning step 6. In practice, the user of the metal detector performs the learning step 6 at the customer location. The learning step 6 begins by passing a sample metal contaminant through aperture 25 at the conveyor line working speed without knowing the conveyor speed Vc. The reactive signal 13 is sampled during the learning procedure 6. If the contaminant passes through the aperture 25 at a linear speed the signal 13 is perceived as a substantially ideal sine wave and the value of Fc can be measured.
Once the value of Fc is determined at step 6, the speed filter control 5 associates the Fc value with whatever value of Vc was entered on the metal detector menu screen. If the speed of the conveyor varies, the speed filter control 5 compensates by adjusting Fc and thereby shifting the combined band pass of the high and low pass filters 4 and 8, respectively, to reflect the change in conveyor speed while still maintaining maximum metal detector sensitivity.
There are numerous disadvantages to the metal calibration method just described. First, there is no method of ensuring that the value for the conveyor speed entered by the operator on the metal detector menu is accurate. Second, many operators find the foregoing calibration procedure to be confusing or overly technical. In general, the operator of the metal detector does not have a clear understanding of the concept and function of the speed filter control 5. This lack of understanding leads to misuse of the speed filter control and necessarily to a reduction in metal detector sensitivity due to improper settings of the metal detector system. A need therefore exists to employ a method of metal detector calibration which does not require the machine operator to deal with the concept of a speed filter control frequency, and which permits substantially all calibration of the metal detector to be performed at the factory prior to use by the customer.